Write Intensive Usage

When copying games from your Steam Library or other very large files (>10 GB), you might have noticed that write speeds on your SSD start out at full speed and then drop considerably. The underlying reason is that modern drives have caches that soak up write bursts to improve performance. In the fairly uncommon scenario of writing data that's too big to fit into these caches, the drive will have to write data directly to flash, and it will probably juggle some out of its write cache at the same time, which can result in a significant loss of write speed. Newer TLC drives use part of their capacity in SLC mode for increased performance. This test can reveal the size of that SLC cache.

Testing on this page looks at exactly that scenario. We write a sequential stream of 1 MB blocks to the drive in a single thread, like a typical file-copy operation would do, and measure write speeds twice a second. The drive is fully erased before testing to ensure any caches are emptied. Please note that this test writes a lot of data in a very short time, which is something most consumers will never do.


Write speeds start out at nearly 3 GB/s, which is super impressive, and stay high until around 475 GB of data have been written—this is a lot! Looks like Silicon Power configured a huge SLC cache for their UD70, which helps the drive soak up incoming bursts of write activity. Since the MPUD70400 is QLC, these ~500 GB mean the drive will use SLC mode until it is completely full. QLC is 4 bits per cell, which turns into only 1 bit per cell in SLC mode—a quarter. Like with all other QLC SSDs, write speeds drop drastically once the SLC cache has been exhausted, down to around 250 MB/s.

Filling the whole drive completes at 259 MB/s, which is a result similar to other QLC-based SSDs, considerably lower than competing TLC drives.

Any pause in write activity, even for a second, will give the drive time to free up some SLC cache, so full write rates are restored after some idle time even if the drive is partially filled.